Up to now, as a heat sink for cooling heating elements such as an IGBT (insulated gate bipolar transistor) used in power electronics, a relatively inexpensive heat sink has been used in many cases, which has a structure in which a base plate having heating members retained on one surface thereof is used, and a plurality of radiating fins are disposed in a comb-shape or a grid-shape on the other surface of the base plate. However, radiating capability of the heat sink having such the configuration is not always sufficient. In particular, in order to enhance cooling capability in a case of effectively cooling semiconductor devices having high heat density, commonly used is a heat sink which uses copper heat pipes having high thermal conductivity or a heat sink which uses an evaporative- or boiling-cooling method in which working fluid in heat pipes is a fluorine-containing compound.
Also, a heat sink (1) in which heat pipes are disposed in the base plate in order to enhance the radiating capability has been proposed in, for example, JP-A-2002-262583 (“JP-A” means unexamined published Japanese patent application). In the proposed heat sink (1), since heat pipes 3 are embedded in a base plate 2, as shown in FIGS. 6(a) and 6(b), heat generated by heating members (4) can be equalized over the entire base plate, and thus the cooling capability is intended to be enhanced by causing the radiating function of radiating fins 5 to act effectively over a wide range.
However, the cooling capability of the heat sinks used in railroad vehicles such as bullet trains (Japan Railway Company's Super-Express, Shinkansen) and automobiles needs to be maintained, even if the heat sinks incline in any direction when the railroad vehicles and automobiles go uphill, take curves, or the like. As in the description of JP-A-2002-262583, since heat pipes are commonly embedded in a plurality of heating members, there is a problem in that, although predetermined cooling capability can be obtained when an inclination occurs in a certain direction, the cooling capability decreases when the inclination occurs in a different direction. This is a fatal problem particularly in the case of the heat sink used in an approximately horizontal state, in which the heat pipes cannot be installed in the inclined state because of a limitation concerning a shape and weight. Further, if a high temperature section is located in the vicinity of a central portion of a base, there is a defect in that the heat pipes do not work effectively.
Meanwhile, the heat sink using the evaporation-cooling method, in which the working fluid for cooling the device having high heat density is a fluorine-containing compound (for example, trade name “Fluorinert FX-3250”, manufactured by Sumitomo 3M Limited), is expensive because an overall configuration of the heat sink is complex. While a refrigerant having low environment load has been developed, a certain type of a fluorine compound having a significant global warming potential is used in many cases as the refrigerant to obtain a relatively good cooling capability. However, there is an increasing hesitation in the usage thereof these days, from the viewpoint of preventing environment degradation.
In addition, in the heat sink using the heat pipes, it is intended that heat should be radiated by conducting the generated heat from the base plate to the radiating fins disposed to the heat pipes. In a case where the heat sink is configured to have a relatively low height so as to be used in train vehicles such as bullet trains, automobiles, or the like, namely, in the case where a length of radiating sections of the heat pipes disposed with the fins is relatively short, there is a disadvantage in that the heat is not discharged sufficiently because effectiveness concerning thermal conductivity from the heat pipes to the radiating fins is insufficient.
Further, in a case where cooling water is used as a cooling fluid, if an ambient temperature in a place where the heat sink having such the configuration is used reaches below 0 (zero) ° C., there is a fear that the cooling water in the heat pipes may be frozen on a radiating section side and not reflow toward a heat-receiving section, so there is a defect in that an essential function of the heat pipes cannot be obtained. In addition, there is a case where the heat pipes themselves are exposed to the outside on a surface of the base plate, so there is a problem in that the exposed portion is broken because the cooling water therein is frozen during use and transportation.